1. Field
This invention relates to apparatus, methods and computer program code for decoding MDCM (modified dual carrier modulation) modulated signals, in particular MDCM modulated OFDM (orthogonal frequency division multiplexed) signals such as those proposed to be used in UWB (ultra wideband) communications systems.
2. Background Art
The MultiBand OFDM (orthogonal frequency division multiplexed) Alliance (MBOA), more particularly the WiMedia Alliance, has published a standard for a UWB physical layer (PHY) for a wireless personal area network (PAN) supporting data rates of up to 480 Mbps. This document was published as, “MultiBand OFDM Physical Layer Specification”, release 1.1, Jul. 14, 2005; release 1.2 is now also available. The skilled person in the field will be familiar with the contents of this document, which are not reproduced here for conciseness. However, reference may be made to this document to assist in understanding embodiments of the invention. Further background material may be found in Standards ECMA-368 and ECMA-369.
Broadly speaking a number of band groups are defined, one at around 3 GHz, a second at around 6 GHz, each comprising three bands; the system employs frequency hopping between these bands in order to reduce the transmit power in any particular band. The OFDM scheme employs 112-122 sub-carriers including 100 data carriers (a total FFT size of 128 carriers) which, at the fastest encoded rate, carry 200 bits using DCM (dual carrier modulation). A ¾ rate Viterbi code results in a maximum data under the current version of this specification of 480 Mbps.
Broadly speaking, in DCM two carriers are employed each using points of a 16 QAM (quadrature amplitude modulation) constellation, but only sixteen combinations of the points are used to encode data—that is, there are only certain allowed combinations of the constellation points on the two carriers.
Details of the UWB DCM modulation scheme can be found in the “MultiBand OFDM physical layer specification” (ibid), in particular at section 6.9.2, which section is hereby incorporated by reference into the present specification.
In detail, a group of two hundred coded and interleaved binary data bits is converted into one hundred complex numbers by grouping the two hundred coded bits into fifty groups of 4 bits each. Each group is represented as (b[g(k)], b[g(k)+1], b[g(k)+50], b[g(k)+51]), where k ε[0,49] and
      g    ⁡          (      k      )        =      {                                        2            ⁢                                                  ⁢            k                                                k            ∈                          [                              0                ,                24                            ]                                                                                      2              ⁢                                                          ⁢              k                        +            50                                                k            ∈                          [                              25                ,                49                            ]                                          Each group of 4 bits is mapped onto a four-dimensional constellation and converted into two complex numbers, d[k] and d[k+50], using the mapping shown in FIG. 1a. The complex numbers are normalised using a normalisation factor of 1/√10. The constellations shown in FIG. 1a can also be expressed using Table 1 below:
TABLE 1d[k]d[k]d[k] + 50d[k] + 50Input BitsI-outQ-outI-outQ-out0000−3−3110001−3−11−30010−31130011−331−10100−1−3−310101−1−1−3−30110−11−330111−13−3−110001−33110011−13−3101011331011133−111003−3−1111013−1−1−3111031−13111133−1−1
We have previously described, in GB0703969.6 and in U.S. Ser. No. 11/715,363 (both hereby incorporated by reference), some improved approaches for decoding DCM modulated data.
An extension to the Wimedia OFDM physical layer (PHY) specification has been proposed which supports data rates of up to 1024 Mbps. For data rates above 480 Mpbs (640 Mbps-1024 Mbps) an LDPC (Low Density Parity Check) forward error correcting code is proposed with coding rates of 1/2, 5/8, 3/4 and 4/5, and the coded data is mapped onto a four-dimensional constellation using modified dual carrier modulation (MCDM).
Details of the planned UWB MDCM modulation scheme can be found in the WiMedia PHY Data Rate Extension Working Text of Jun. 18, 2008, in particular at section 6.9.3, which section is hereby incorporated by reference into the present specification. This material may in future appear at section 6.9.3 of the “MultiBand OFDM physical layer specification” (ibid), which would be an additional section in this document.
In detail, the LDPC coded data is grouped into sets of 400 bits and each of these sets is converted into a set of 100 complex numbers using MCDM and modulated onto a symbol.
The 400 bits modulated on each symbol are grouped into 50 groups of 8 bits. Each group is represented as b[8×k],b[8×k+1], . . . ,b[8×k+7] where k ε [0,49], and each group of 8 bits is converted into two complex numbers (d[k], d[k+50]). These values are generated by means of two intermediate values xa and xb which are determined by the values in Table 2 below. The values xa and xb are generated as Gray coded 16 QAM constellation points, selected for xa by the input bits b[8×k], . . . ,b[8×k+3] and for xb by bits b[8×k+4], . . . , b[8×k+7]. The (complex OFDM subcarrier) values d[k] and d[k+50] are then generated using:
            (                                                  d              ⁡                              [                k                ]                                                                                        d              ⁡                              [                                  k                  +                  50                                ]                                                        )        =                  (                                            4                                      1                                                          1                                                      -                4                                                    )            ×              (                                                            x                a                                                                                        x                b                                                    )            ×              1        /                  170                      ,where 1/√{square root over (170)} is a normalisation factor.
TABLE 2input bitsI-outQ-out0000330001130010−330011−130100310101110110−310111−1110003−310011−31010−3−31011−1−311003−111011−11110−3−11111−1−1
There is a need for decoding techniques for MCDM coded data.